I. Field of the Invention
The present invention relates generally to antennas, and in particular to a low profile, wideband, GNSS dual frequency antenna structure.
II. Description of the Related Art
Various antenna designs and configurations have been produced for transmitting and receiving electromagnetic (wireless) signals. Antenna design criteria include the signal characteristics and the applications of the associated equipment, i.e. transmitters and receivers. For example, stationary, fixed applications involve different antenna design configurations than mobile equipment.
Global navigation satellite systems (GNSSs) have progressed within the last few decades to their present state-of-the-art, which accommodates a wide range of positioning, navigating and informational functions and activities. GNSS applications are found in many industries and fields of activity. For example, navigational and guidance applications involve portable GNSS receivers ranging from relatively simple, consumer-oriented, handheld units to highly sophisticated airborne and marine vessel equipment.
Vehicle-mounted antennas are designed to accommodate vehicle motion, which can include movement in six degrees of freedom, i.e. pitch, roll and yaw corresponding to vehicle rotation about X, Y and Z axes in positive and negative directions respectively. Moreover, variable and dynamic vehicle attitudes and orientations necessitate antenna gain patterns which provide GNSS ranging signal strengths throughout three-dimensional ranges of motion corresponding to the vehicles' operating environments. For example, aircraft in banking maneuvers often require below-horizon signal reception. Ships and other large marine vessels, on the other hand, tend to operate relatively level and therefore normally do not require below-horizon signal acquisition. Terrestrial vehicles have varying optimum antenna gain patterns dependent upon their operating conditions. Agricultural vehicles and equipment, for example, often require signal reception in various attitudes in order to accommodate operations over uneven terrain. Modern precision agricultural GNSS guidance equipment, e.g., sub-centimeter accuracy, requires highly efficient antennas which are adaptable to a variety of conditions.
Another antenna/receiver design consideration in the GNSS field relates to multipath interference, which is caused by reflected signals that arrive at the antenna out of phase with the direct signal. Multipath interference is most pronounced at low elevation angles, e.g., from about 10° to 20° above the horizon. They are typically reflected from the ground and ground-based objects. Antennas with strong gain patterns at or near the horizon are particularly susceptible to multipath signals, which can significantly interfere with receiver performance based on direct line-of-sight (LOS) reception of satellite ranging signals and differential correction signals (e.g., DGPS). Therefore, important GNSS antenna design objectives include achieving the optimum gain pattern, balancing rejecting multipath signals and receiving desired ranging signals from sources, e.g., satellites and pseudolites, at or near the horizon.
The present invention addresses these objectives by providing GNSS antennas with selectable gain patterns. For example, a wide beamwidth with tracking capability below the horizon is possible with a taller central support mounting a radiating element arm assembly of a crossed-dipole antenna. A wide beamwidth is preferred for vehicles which have significant pitch and roll, such as aircraft and small watercraft. By reducing the height of the central support structure a much steeper roll off at the horizon is generated with attenuated back lobes, which is preferred for maximal multipath rejection in high accuracy applications. Such alternative configurations can be accommodated by changing the height of the support element, which is preferably designed and built for assembly in multiple-height configurations depending upon the particular intended antenna applications.
Another beamwidth-performance variable relates to the deflection or “droop” of the crossed-dipole radiating element arms, which can range from nearly horizontal to a “full droop” position attached at their ends to a ground plane. Wider beam widths are achieved by increasing the downward deflection whereas multipath rejection is enhanced by decreasing droop. Preferably a selectable gain antenna accommodates such alternative configurations without significantly varying the input impedance whereby common matching and phasing networks can be used for all applications.
A typical approach to construct a dual frequency low profile antenna is to use stacked patches constructed of ceramic material with a dielectric constant of approximately 10. This approach typically results in a compact antenna, but due to the relatively high dielectric constant the bandwidth is quite narrow, which compromises reception performance for both Global Positioning System (GPS), GLONASS (Russian navigation satellite system) and other global navigation satellite systems (GNSSs), unless the ceramic is very thick. This increases the cost and creates issues with coupling between both patches, making it difficult to get the right gain pattern and polarization. A further issue is the use of a single feed point on both patches to minimize the impact of the feed for the top element passing through the second element. This relies on a dual resonance patch and the phase difference of this dual resonance to be exactly 90 degrees at the center frequency. This further limits the bandwidth where the antenna operates with the correct polarization.
Heretofore there has not been available an antenna with the advantages and features of the present invention.